Modern gas turbines often operate at extremely high temperatures. The effect on the turbine blades and/or stator vanes can be detrimental to the efficient operation of the turbine and can, in extreme circumstances, lead to distortion and possible failure of the blade or vane. In order to overcome this risk, high temperature turbines may include hollow blades or vanes incorporating so-called impingement tubes. These are hollow tubes that run radially within the blades or vanes. Cooling air is forced into and along these tubes and emerges through suitable apertures into the void between the tubes and the inner surfaces of the hollow blades or vanes. Air expelled from the apertures impinges on the inner surfaces of the hollow blades or vanes (so-called “impingement cooling”) and also creates an internal airflow to cool the blade or vane.
Normally, blades and vanes are made by casting. Impingement tubes may be inserted into the hollow structure from one or the other end and welded or otherwise fixed in place. Chordwise extending ribs are also often cast inside the blades, mainly to direct coolant and to provide a greater cooling surface area. These ribs, or selected of them, may serve as location spacers for the impingement tubes, so as to create the necessary internal space for the cooling air.
Problems arise with fitting impingement tubes into the latest generation of blades or vanes in that the aerofoil sections of the blades or vanes may be extremely complicated. Hollow aerofoils may feature multidirectional curvature. In some designs, the mid-section may actually be smaller than either the tip or hub sections. A technique for enabling an impingement tube to be fitted inside such a hollow turbine blade or vane is therefore a necessity.